Subsea wells typically connect to a subsea manifold that delivers the well fluid to a production platform for processing, particularly for the removal of water and gas. The oil is then transmitted to a pipeline or other facility for export from the production platform. Production of fluids from a medium to deep subsea environment requires compensation for the effects of cold temperatures, high ambient pressures and fluid viscosity as a function of break out of gas in the fluid stream. In flowing wells, particularly those with light API fluid, these conditions may be mitigated by the nature of the producing reservoir. In wells with low API oil and insufficient pressure to drive the fluid to the surface, some form of artificial lift will be required.
One type of artificial lift for wells employs an electrical submersible pump, which is a type that has been used for many years on land based wells. An electrical submersible pump typically has an electrical motor, a rotary pump and a seal section located between the pump and the motor for equalizing hydrostatic fluid pressure with the internal pressure of lubricant in the motor. These types of pumps must be retrieved periodically for repair or replacement due to normal wear, as often as every eighteen months.
Pulling a pump to replace it normally requires a workover rig, because most pumps are suspended on strings of tubing. Pulling production tubing on an offshore well is much more expensive than a land-based or surface wellhead. An intervention to remove the pump of an offshore well must be scheduled months in advance, depending on the production method. The cost, coupled with lost production, will in some cases make large potential reservoirs non-economical.
There have been proposals to utilize pumps at the seafloor to pump the well fluid flowing from the well to the sea floor level. A number of problems are associated with the task, including periodically replacing the pump from the seafloor without the need for an expensive workover or drilling rig. One factor to consider is that the sea cannot be polluted with well fluid, thus traditionally risers have been employed during drilling and intervention operations that shield sea water from well components as they are pulled to the surface. If a riser must be employed to remove and replace a seafloor or mudline pump, a workover rig must still be employed at a great expense.